1. Field of the Invention
The invention relates to an incremental angle encoder having a first disk with a graduation and a first and a second reference mark connected to a second disk having a spiral shaped reference scanning field thereon. A first reference signal is generated by scanning the first reference mark and a second reference signal is generated by scanning the second reference mark through the reference scanning field on the second disk. The number of revolutions of the encoder shaft is determined by the offset between the two reference signals.
2. Description of Prior Art
From EP-Al-0 300 183 an incremental angle encoder is known, having a disk with an incremental angular graduation and a reference mark, to which a two-stage absolute value encoder is coupled by means of a reduction gear, which records every revolution of the incremental angle encoder by means of a reference signal derived from the reference mark. A logic unit generates an additional reference signal from the reference signal of the incremental angle encoder, which, in accordance with the rpm value from the signals of the absolute value encoder, is offset from the reference signal of the incremental angle encoder. A processor determines the absolute angular measurement value from the increment signal of the incremental angle encoder, the signals of the absolute value encoder and the two reference signals.
This type of incremental angle encoder has the disadvantage that the system is large and expensive. For example, a two-stage, 10-bit absolute value encoder requires eleven photoelements and signal generators.
Another incremental angle encoder is described in German Published, Non-examined Patent Application DE-OS 34 16 090. A first disk having an incremental angular graduation and a reference mark is connected to a second disk with a second reference mark via a reduction gear. The reduction is chosen in such a manner that for every revolution of the encoder shaft with the first disk there is a specific offset between the two reference signals derived from the two reference marks. The absolute angular measurement value is determined from the incremental signal of the first disk and the specific offset between the two reference signals. This incremental angle encoder, however, has the disadvantage that the accurately of the offset between the two reference signals depends upon the accuracy of the reduction gear. This arrangement requires either great expenditure in the reduction gear or a toleration of a smaller number of distinguishable revolutions of the first disk.